The rate of pulmonary gas exchange is affected by many factors but the two principal processes which appear to limit the kinetics of 02 and C02 exchange are the duration of capillary transit time and the rates of intracapillary reactions of the respiratory gases. Therefore the applicant proposes to study the variation in transit times throughout different segments of the pulmonary vasculature and the rate of gas exchange in the intact circulation of the lung. The kinetics of gas exchange would be determined in an isolated, perfused lung preparation contained within a plethysmograph. The lungs would be perfused with buffer by a hydraulic pump which also removes water from the plethysmograph at the same rate to prevent any net volume change. A 2.0 ml bolus of reactant would be injected into the perfusion system. Subsequent gas exchange produces a pressure transient which is related to the corresponding volume of exchanged gas. With knowledge of the concentration profile of a bolus injection, the observed exchange of an inert gas dissolved in a bolus, and the rate of oxygen exchange following injection of a bolus of sodium dithionite, the applicant would determine the distribution of transit times in the arterial and capillary segments of the pulmonary circulation. Injection of suspensions of erythrocytes would indicate the rate of capillary gas exchange.